Astronomers have detected S301, the fastest known star in the Milky Way, racing around the supermassive black hole Sagittarius A* at a staggering 15,500 miles per second—over 8% the speed of light—providing a probe to test Einstein’s theory of general relativity.
Spotted using high-resolution interferometry, this object is altering how researchers approach the gravitational mechanics governing the core of our galaxy.
The Mechanics of an Extreme Orbit
S301 moves along a highly elongated, tightly bound orbital path around Sagittarius A*, the 4.3-million-solar-mass black hole anchoring the Milky Way. According to findings published on August 19 in the journal Nature, the star completes a full orbit in just 8.7 years. At its closest approach, S301 sweeps past the supermassive black hole at a distance roughly 12 times the gap between Earth and the sun.
Felix Mang, a Ph.D. student at the Max Planck Institute for Extraterrestrial Physics in Germany, highlighted the unprecedented nature of these orbital parameters in a statement, noting the proximity and velocity metrics involved. The star achieves speeds reaching about 15,500 miles, or 25,000 kilometers, per second.
In 2023, scientists identified S301 utilizing the GRAVITY instrument mounted on the European Southern Observatory’s Very Large Telescope Interferometer located in Chile. By tracing these observations back through 2021 and 2017, the team mapped out the star’s trajectory. Analysis suggests S301 may once have belonged to a binary system that wandered too close to Sagittarius A*, allowing the black hole to capture the star while flinging its companion outward at tremendous speed.
Probing Spacetime and Black Hole Spin
The discovery of S301 serves as a laboratory for fundamental physics. General relativity predicts that a rotating black hole drags spacetime along with it—a phenomenon known as frame dragging, or Lense-Thirring precession. Because S301 ventures so close to Sagittarius A*, this spatial distortion may exert a measurable shift on the star’s orbit over time.
The wider consequences for theoretical physics were highlighted by Stefan Gillessen, a co-author of the study at the Max Planck Institute, who noted that this interaction provides a means to directly measure the spin of a massive black hole, serving as a critical test for Einstein’s theory. Researchers anticipate that these changes may become measurable within about a decade.
To capture these orbital adjustments, astronomers are planning follow-up tracking initiatives. Researchers intend to leverage the GRAVITY+ instrument suite and, ultimately, the European Southern Observatory’s Extremely Large Telescope. With the star’s next close pass projected for 2031, observations spanning two full orbits could reveal Sagittarius A*’s spin for the first time.
Observational Parameters at the Galactic Core
- Target Object: S301, a faint star orbiting the galactic center
- Host Black Hole: Sagittarius A* (estimated at 4.3 million solar masses)
- Peak Velocity: Approximately 15,500 miles (25,000 kilometers) per second (>8% speed of light)
- Orbital Period: 8.7 Earth years
- Primary Instrument: GRAVITY on the Very Large Telescope Interferometer (VLTI)
- Publication Venue: Nature (August 19)
Decoding the kinematics of stars orbiting a supermassive black hole requires pushing optical and infrared interferometry to its limits. As instrumentation like the European Southern Observatory infrastructure scales in resolution, the mechanics governing galactic evolution are coming into focus.
